Method of producing semiconductor epitaxial wafer, semiconductor epitaxial wafer, and method of producing solid-state image sensing device

ABSTRACT

The present invention provides a method of more efficiently producing a semiconductor epitaxial wafer, which can suppress metal contamination by achieving higher gettering capability. 
     A method of producing a semiconductor epitaxial wafer  100  according to the present invention includes a first step of irradiating a semiconductor wafer  10  with cluster ions  16  to form a modifying layer  18  formed from a constituent element of the cluster ions  16  in a surface portion  10 A of the semiconductor wafer; and a second step of forming an epitaxial layer  20  on the modifying layer  18  of the semiconductor wafer  10.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.14/116,318, filed Nov. 7, 2013, which is the National Stage ofInternational Patent Application No. PCT/JP2012/001892, filed Mar. 19,2012, the entire disclosures of which are expressly incorporated byreference herein.

TECHNICAL FIELD

The present invention relates to a method of producing a semiconductorepitaxial wafer, a semiconductor epitaxial wafer, and a method ofproducing a solid-state image sensing device. The present inventionparticularly relates to a method of more efficiently producing asemiconductor epitaxial wafer, which can suppress metal contamination byachieving higher gettering capability.

BACKGROUND ART

Metal contamination is one of the factors that deteriorate thecharacteristics of a semiconductor device. For example, for aback-illuminated solid-state image sensing device, metal mixed into asemiconductor epitaxial wafer to be a substrate of the device causesincreased dark current in the solid-state image sensing device, andresults in formation of defects referred to as white spot defects. Inrecent years, back-illuminated solid-state image sensing devices havebeen widely used in digital video cameras and mobile phones such assmartphones, because they can directly receive light from the outside,and take sharper images or motion pictures even in dark places and thelike due to the fact that a wiring layer and the like thereof aredisposed at a lower layer than a sensor section. Therefore, it isdesirable to reduce white spot defects as much as possible.

Mixing of metal into a wafer mainly occurs in a process of producing asemiconductor epitaxial wafer and a process of producing a solid-stateimage sensing device (device fabrication process). Metal contaminationin the former process of producing a semiconductor epitaxial wafer maybe due to heavy metal particles from components of an epitaxial growthfurnace, or heavy metal particles caused by metal corrosion of pipingmaterials of the furnace due to chlorine-based gas used in epitaxialgrowth in the furnace. In recent years, such metal contaminations havebeen reduced to some extent by replacing components of epitaxial growthfurnaces with highly corrosion resistant materials, but not to asufficient extent. On the other hand, in the latter process of producinga solid-state image sensing device, heavy metal contamination ofsemiconductor substrates would occur in process steps such as ionimplantation, diffusion, and oxidizing heat treatment in the producingprocess.

For these reasons, conventionally, heavy metal contamination ofsemiconductor epitaxial wafers has been avoided by forming, in thesemiconductor wafer, a gettering sink for trapping the metal, or using asubstrate, such as a high boron concentration substrate, having highability to trap the metal (gettering capability).

In general, a gettering sink is formed in a semiconductor wafer by anintrinsic gettering (IG) method in which an oxygen precipitate (commonlycalled a silicon oxide precipitate, and also called BMD: bulk microdefect) or dislocation that are crystal defects is formed within thesemiconductor wafer, or an extrinsic gettering (EG) method in which thegettering sink is formed on the rear surface of the semiconductor wafer.

Here, a technique of forming a gettering site in a semiconductor waferby ion implantation can be given as a technique for gettering heavymetal. JP H06-338507 A (PTL 1) discloses a producing method, by whichcarbon ions are implanted through a surface of a silicon wafer to form acarbon ion implanted region, and a silicon epitaxial layer is formed onthe surface thereby obtaining a silicon epitaxial wafer. In thattechnique, the carbon ion implanted region functions as a getteringsite.

Further, JP 2008-294245 A (PTL 2) discloses a method of forming a carbonimplanted layer by implanting carbon ions into a silicon wafer, and thenperforming heat treatment using an RTA (Rapid Thermal Annealing)apparatus for recovering the crystallinity of the wafer which has beendisrupted by the ion implantation, thereby shortening the recovery heattreatment process.

CITATION LIST Patent Literature

-   PTL 1: JP H06-338507 A-   PTL 2: JP 2008-294245 A

SUMMARY OF INVENTION Technical Problem

In both of the techniques described in PTL 1 and PTL 2, single ions areimplanted in a semiconductor wafer before forming an epitaxial layer.However, according to studies made by the inventors of the presentinvention, it was found that white spot defects cannot be sufficientlyreduced in solid-state image sensing devices produced usingsemiconductor epitaxial wafers subjected to single-ion implantation, andthe semiconductor epitaxial wafers are required to achieve strongergettering capability.

Further, since implantation of single ions greatly disruptscrystallinity of a semiconductor wafer, it is required to perform heattreatment on the semiconductor wafer for recovering the crystallinitythereof (hereinafter referred to as “recovery heat treatment”) at a hightemperature for a long time before forming an epitaxial layer. However,a long-time recovery heat treatment at a high temperature affects theimprovement in the throughput. In PTL 2, the time for a recovery heattreatment itself is shortened, but the recovery heat treatment isperformed using an RTA apparatus separate from the epitaxial apparatus,which also makes it impossible to achieve high throughput.

In view of the above problems, an object of the present invention is toprovide a method of more efficiently producing a semiconductor epitaxialwafer, which can suppress metal contamination by achieving highergettering capability.

Solution to Problem

According to further studies made by the inventors of the presentinvention, irradiation of an epitaxial layer formed on a semiconductorwafer with cluster ions is advantageous in the following points ascompared with implantation of single ions into the epitaxial layer.Specifically, even if irradiation with cluster ions is performed at anacceleration voltage equivalent to the case of single ion implantation,the energy per one atom or one molecule applied to the irradiatedsemiconductor wafer is lower than in the case of single ionimplantation. This results in higher peak concentration in the depthdirection profile of the irradiation element, and allows the peakposition to approach the surface of the semiconductor wafer. Thus, thegettering capability was found to be improved. Further, it was foundthat irradiation with cluster ions can reduce damage to crystals ascompared with single-ion implantation, which makes it possible to omitrecovery heat treatment performed after the ion irradiation. Based onthe above findings, the inventors completed the present invention.

Specifically, a method of producing a semiconductor epitaxial waferaccording to the present invention characteristically includes a firststep of irradiating a semiconductor wafer with cluster ions to form amodifying layer formed from a constituent element of the cluster ions ina surface portion of the semiconductor wafer; and a second step offorming an epitaxial layer on the modifying layer of the semiconductorwafer.

Here, the semiconductor wafer may be a silicon wafer.

Further, the semiconductor wafer may be an epitaxial silicon wafer inwhich a silicon epitaxial layer is formed on a surface of a siliconwafer. In this case, the modifying layer is formed in a surface portionof the silicon epitaxial layer in the first step.

In the present invention, the second step can be performed aftertransferring the semiconductor wafer into an epitaxial growth apparatus,without heat treating the semiconductor wafer for recovering thecrystallinity after the first step.

Here, the cluster ions preferably contain carbon as a constituentelement. More preferably, the cluster ions contain at least two kinds ofelements including carbon as constituent elements.

Further, in the first step, the semiconductor wafer can be irradiatedwith the cluster ions such that the peak of a concentration profile ofthe constituent element in the depth direction of the modifying layerlies at a depth within 150 nm from the surface of the semiconductorwafer.

The first step is preferably performed under the conditions of: theacceleration voltage of cluster ions is less than 100 keV/Cluster, thecluster size is 100 or less, and the cluster dose is 1×10¹⁶ atoms/cm² orless. Further, the first step is more preferably performed under theconditions of: the acceleration voltage of cluster ions is 80keV/Cluster or less, the cluster size is 60 or less, and the clusterdose is 5×10¹³ atoms/cm² or less.

Next, a semiconductor epitaxial wafer according to the present inventionincludes a semiconductor wafer; a modifying layer formed from a certainelement contained as a solid solution in the semiconductor wafer, in asurface portion of the semiconductor wafer; and an epitaxial layer onthe modifying layer. The half width of a concentration profile of thecertain element in the depth direction of the modifying layer is 100 nmor less.

Here, the semiconductor wafer may be a silicon wafer.

Further, the semiconductor wafer may be an epitaxial silicon wafer inwhich a silicon epitaxial layer is formed on a surface of a siliconwafer. In this case, the modifying layer is located in a surface portionof the silicon epitaxial layer.

Moreover, the peak of the concentration profile in the modifying layerpreferably lies at a depth within 150 nm from the surface of thesemiconductor wafer. The peak concentration of the concentration profilein the modifying layer is preferably 1×10¹⁵ atoms/cm³ or more.

Here, the certain element preferably includes carbon. More preferably,the certain element includes at least two kinds of elements includingcarbon.

In a method of producing a solid-state image sensing device according tothe present invention, a solid-state image sensing device is formed onthe epitaxial layer located in the surface portion of the epitaxialwafer fabricated by any one of the above producing methods or of any oneof the above epitaxial wafers.

Advantageous Effect of Invention

According to the method of producing a semiconductor wafer of thepresent invention, a semiconductor wafer is irradiated with clusterions, and a modifying layer is formed from (a) constituent element(s) ofthe cluster ions in a surface portion of the semiconductor wafer, whichallows the modifying layer to achieve higher gettering capability. Thus,a semiconductor epitaxial wafer which makes it possible to suppressmetal contamination can be produced. Further, the irradiation withcluster ions can reduce damage to crystals as compared with single-ionimplantation, which makes it possible to omit recovery heat treatmentperformed after the ion irradiation. Thus, the semiconductor epitaxialwafer can be more sufficiently produced.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A, 1B, and 1C are schematic cross-sectional views illustrating amethod of producing a semiconductor epitaxial wafer 100 according to anembodiment of the present invention.

FIGS. 2A, 2B, 2C, and 2D are schematic cross-sectional viewsillustrating a method of producing a semiconductor epitaxial wafer 200according to another embodiment of the present invention.

FIG. 3A is a schematic view illustrating the irradiation mechanism forirradiation with cluster ions. FIG. 3B is a schematic view illustratingthe implantation mechanism for implantation with single ions.

FIG. 4 is a graph showing the distribution of the carbon concentrationwith respect to the depth from the surface of a silicon wafer(concentration profile) in Example 1-2 and Comparative Example 1-2.

FIG. 5 shows graphs for comparing the Ni gettering capability of Example1-2 and Comparative Example 1-2.

FIG. 6 shows graphs for comparing the Cu gettering capability of Example1-2 and Comparative Example 1-2.

FIG. 7 shows LPD maps of Example 1-2 and Comparative Example 1-2.

FIG. 8 is a graph showing the distribution of the carbon concentrationwith respect to the depth from the surface of a silicon wafer(concentration profile) in Example 2-2 and Comparative Example 2-2.

FIG. 9 shows graphs for comparing the Ni gettering capability of Example2-2 and Comparative Example 2-2.

FIG. 10 shows graphs for comparing the Cu gettering capability ofExample 2-2 and Comparative Example 2-2.

FIG. 11 shows LPD maps of Example 2-2 and Comparative Example 2-2.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below in detailwith reference to the drawings. In principle, the same components aredenoted by the same reference numeral, and the description will not berepeated. Further, in FIGS. 1A, 1B, and 1C and FIGS. 2A, 2B, 2C, and 2D,a first epitaxial layer 14 and a second epitaxial layer 20 areexaggerated with respect to a semiconductor wafer 10 in thickness forthe sake of explanation, so the thickness ratio does not conform to theactual ratio.

(Method of Producing Semiconductor Epitaxial Wafer)

A method of producing a semiconductor epitaxial wafer 100 according to afirst embodiment of the present invention includes, as shown in FIGS.1A, 1B, and 1C, a first step (FIGS. 1A and 1B) of irradiating asemiconductor wafer 10 with cluster ions 16 to form a modifying layer 18formed from a constituent element of the cluster ions 16 in a surfaceportion 10A of the semiconductor wafer 10; and a second step (FIG. 1C)of forming an epitaxial layer 20 on the modifying layer 18 of thesemiconductor wafer 10. FIG. 1C is a schematic cross-sectional view ofthe semiconductor epitaxial wafer 100 obtained by this producing method.

Examples of the semiconductor wafer 10 include, for example, a bulksingle crystal wafer including silicon or a compound semiconductor(GaAs, GaN, or SiC) with no epitaxial layer on the surface thereof. Ingeneral, a bulk single crystal silicon wafer is used in cases ofproducing back-illuminated solid-state image sensing devices. Further,the semiconductor wafer 10 may be prepared by growing a single crystalsilicon ingot by the Czochralski process (CZ process) or floating zonemelting process (FZ process) and slicing it with a wire saw or the like.Further, carbon and/or nitrogen may be added thereto to achieve highergettering capability. Furthermore, the semiconductor wafer 10 may bemade n-type or p-type by adding certain impurities. The first embodimentshown in FIGS. 1A, 1B, and 1C is an example of using a bulksemiconductor wafer 12 with no epitaxial layer on its surface, as thesemiconductor wafer 10.

Alternatively, an epitaxial semiconductor wafer in which a semiconductorepitaxial layer (first epitaxial layer) 14 is formed on a surface of thebulk semiconductor wafer 12 as shown in FIG. 2A, can be given as anexample of the semiconductor wafer 10. For example, an epitaxial siliconwafer in which a silicon epitaxial layer is formed on a surface of abulk single crystal silicon wafer can be given. The silicon epitaxiallayer may be formed by CVD process under typical conditions. The firstepitaxial layer 14 preferably has a thickness in the range of 0.1 μm to10 μm, more preferably in the range of 0.2 μm to 5 μm.

A method of producing a semiconductor epitaxial wafer 200 according to asecond embodiment of the present invention includes, as shown in FIGS.2A, 2B, 2C, and 2D, a first step (FIGS. 2A, 2B, and 2C) of irradiating asemiconductor wafer 10, in which a first epitaxial layer 14 is formed ona surface (at least one side) of the bulk semiconductor wafer 12, withcluster ions 16 to form a modifying layer 18 formed from a constituentelement of the cluster ions 16 in a surface portion 10A of thesemiconductor wafer 10 (the surface portion of the first epitaxial layer14 in this embodiment); and a second step (FIG. 2D) of forming anepitaxial layer 20 on the modifying layer 18 of the semiconductor wafer10. FIG. 2D is a schematic cross-sectional view of the semiconductorepitaxial wafer 200 obtained by this producing method.

Here, the step of irradiation with cluster ions shown in FIG. 1A andFIG. 2B is a characteristic step of the present invention. The technicalmeaning of employing the characteristic step will be described with theoperation and effect. The modifying layer 18 formed as a result ofirradiation with the cluster ions 16 is a region where the constituentelement of the cluster ions 16 is localized as a solid solution atcrystal interstitial positions or substitution positions in the crystallattice of the surface portion of the semiconductor wafer, which regionfunctions as a gettering site. The reason may be as follows. Afterirradiation in the form of cluster ions, elements such as carbon andboron are localized at high density at substitution positions andinterstitial positions in the silicon single crystal. It has beenexperimentally found that when carbon or boron are turned into solidsolutions to the equilibrium concentration of the silicon single crystalor higher, the solid solubility of heavy metals (saturation solubilityof transition metal) extremely increases. It is considered that carbonor boron made into solid solutions to the equilibrium concentration orhigher increases the solubility of heavy metals, which results insignificantly increased rate of trapping the heavy metals.

Here, since irradiation with the cluster ions 16 is performed in thepresent invention, higher gettering capability can be achieved ascompared to cases of implanting single ions, and recovery heat treatmentcan be omitted. Therefore, the semiconductor epitaxial wafers 100 and200 achieving higher gettering capability can be more efficientlyproduced, and even less white spot defects occur in back-illuminatedsolid-state image sensing devices produced from the semiconductorepitaxial wafers 100 and 200 obtained by the producing methods. Notethat “cluster ions” herein mean clusters formed by aggregation of aplurality of atoms or molecules, which are ionized by being positivelyor negatively charged. A cluster is a bulk aggregate having a plurality(typically 2 to 2000) of atoms or molecules bound together.

The inventors of the present invention consider the operation achievingsuch an effect as follows.

For example, when carbon single ions are implanted into a silicon wafer,the single ions sputter silicon atoms forming the silicon wafer to beimplanted to a predetermined depth position in the silicon wafer, asshown in FIG. 3B. The implantation depth depends on the kind of theconstituent element of the implantation ions and the accelerationvoltage of the ions. In this case, the concentration profile of carbonin the depth direction of the silicon wafer is relatively broad, and thecarbon implanted region extends approximately 0.5 μm to 1 μm. Whenimplantation is performed simultaneously with a plurality of species ofions at the same energy, lighter elements are implanted more deeply, inother words, elements are implanted at different positions depending ontheir masses. Accordingly, the concentration profile of the implantedelements is broader in such a case. Further, in the formation of anepitaxial layer after ion implantation, the implanted elements arediffused due to heat, which is also a factor of the broaderconcentration profile.

Single ions are typically implanted at an acceleration voltage of about150 keV to 2000 keV. However, since the ions collide with silicon atomswith the energy, which results in disrupted crystallinity of the surfaceportion of the silicon wafer, to which the single ions are implanted.Accordingly, the crystallinity of an epitaxial layer to be grown lateron the wafer surface is disrupted. Further, the higher the accelerationvoltage is, the more the crystallinity is disrupted. Therefore, it isrequired to perform heat treatment for recovering the crystallinityhaving been disrupted at a high temperature for a long time after ionimplantation (recovery heat treatment).

On the other hand, when cluster ions formed from, for example, carbonand boron are irradiated into a silicon wafer as shown in FIG. 3A, thecluster ions 16 are instantaneously turned into a high temperature stateof about 1350° C. to 1400° C. due to the irradiation energy, thusmelting silicon. After that, the silicon is rapidly cooled to form solidsolutions of carbon and boron in the vicinity of the surface of thesilicon wafer. Accordingly, a “modifying layer” herein means a layer inwhich the constituent element of the ion used for irradiation forms asolid solution at crystal interstitial positions or substitutionpositions in the crystal lattice of the surface portion of thesemiconductor wafer. When the concentration profile of the constituentelement in the depth direction of the silicon wafer is measured usingSIMS, the “modifying layer” is also specified as a zone where theconstituent element is detected more in the background. Theconcentration profile of carbon and boron in the depth direction of thesilicon wafer is sharper as compared with the case of single ions,although depending on the acceleration voltage and the cluster size ofthe cluster ions. The region where carbon and boron are irradiated is aregion of 500 nm or less (for example, about 50 nm to 400 nm). Further,as compared with monomer ions, since the ions to be irradiated formclusters, the cluster ions are not channeled through the crystallattice, and thermal diffusion of constituent elements is suppressed,which leads to the sharp concentration profile. Consequently, carbon andboron are precipitated at a high concentration in a localized region.Since the modifying layer 18 is formed in the vicinity of the surface ofthe silicon wafer, further proximity gettering can be performed. This isconsidered to result in achievement of still higher getteringcapability. Note that, when irradiation is performed simultaneously witha plurality of species of ions in the form of cluster ions, these ionsare not implanted to different depths, which is preferable because theconstituent element of the irradiated ions can be located in thevicinity of the surface.

In general, irradiation with cluster ions is performed at anacceleration voltage of about 10 keV/Cluster to 100 keV/Cluster.However, since a cluster is an aggregate of a plurality of atoms ormolecules, the ions can be irradiated at reduced energy per one atom orone molecule, which reduces damage to the crystals in the semiconductorwafer. Further, cluster ion irradiation does not disrupt thecrystallinity of a semiconductor wafer as compared with single-ionimplantation also due to the above described implantation mechanism.Accordingly, after the first step, without performing recovery heattreatment on the semiconductor wafer 10, the semiconductor wafer 10 canbe transferred into an epitaxial growth apparatus to be subjected to thesecond step.

The cluster ions 16 may include a variety of clusters depending on thebinding mode, and can be generated, for example, by known methodsdescribed in the following documents. Methods of generating gas clusterbeam are described in (1) JP 09-041138 A and (2) JP 04-354865 A. Methodsof generating ion beam are described in (1) Junzo Ishikawa, “Chargedparticle beam engineering”, ISBN 978-4-339-00734-3 CORONA PUBLISHING,(2) The Institution of Electrical Engineers of Japan, “Electron/Ion BeamEngineering”, Ohmsha, ISBN 4-88686-217-9, and (3) “Cluster IonBeam—Basic and Applications”, THE NIKKAN KOGYO SHIMBUN, ISBN4-526-05765-7. In general, a Nielsen ion source or a Kaufman ion sourceis used for generating positively charged cluster ions, whereas a highcurrent negative ion source using volume production is used forgenerating negatively charged cluster ions.

The conditions for irradiation with cluster ions will be describedbelow. First, examples of the element used for irradiation include, butnot limited to, carbon, boron, phosphorus, and arsenic. However, interms of achieving higher gettering capability, the cluster ionspreferably contain carbon as a constituent element. Carbon atoms at alattice site have a smaller covalent radius than silicon singlecrystals, so that a compression site is produced in the silicon crystallattice, which results in high gettering capability for attractingimpurities in the lattice.

Further, the cluster ions preferably contain at least two kinds ofelements including carbon as constituent elements. Since the kinds ofmetals to be efficiently gettered depend on the kinds of theprecipitated elements, solid solutions of two or more kinds of elementscan cover a wider variety of metal contaminations. For example, carboncan efficiently getter nickel, whereas boron can efficiently gettercopper and iron.

The compounds to be ionized are not limited in particular, but examplesof compounds to be suitably ionized include ethane, methane, propane,benzyl gas (C₇H₇), and carbon dioxide (CO₂) as carbon sources, anddiborane and decaborane gas (B₁₀H₁₄) as boron sources. For example, whena mixed gas of benzyl gas and decaborane gas is used as a material gas,a hydrogen compound cluster in which carbon, boron, and hydrogen areaggregated can be produced. Alternatively, when cyclohexane (C₆H₁₂) isused as a material gas, cluster ions formed from carbon and hydrogen canbe produced.

Next, the acceleration voltage and the cluster size of the cluster ionsare controlled, thereby controlling the peak position of theconcentration profile of the constituent elements in the depth directionof the modifying layer 18. “Cluster size” herein means the number ofatoms or molecules constituting one cluster.

In the first step of this embodiment, in terms of achieving highergettering capability, the irradiation with the cluster ions 16 isperformed such that the peak of the concentration profile of theconstituent elements in the depth direction of the modifying layer 18lies at a depth within 150 nm from the surface of the semiconductorwafer 10. Note that in this specification, “the concentration profile ofthe constituent elements in the depth direction” in the case where theconstituent elements include at least two kinds of elements, means theprofiles with respect to the respective single element but not withrespect to the total thereof.

For a condition required to set the peak positions to the depth level,the acceleration voltage of the cluster ions is set at higher than 0keV/Cluster and less than 100 keV/Cluster, preferably at 80 keV/Clusteror less, and more preferably 60 keV/Cluster or less. Further, thecluster size is 2 to 100, preferably 60 or less, more preferably 50 orless.

In addition, for adjusting the acceleration voltage, two methods of (1)electrostatic field acceleration and (2) oscillating field accelerationare commonly used. The former method includes a method in which aplurality of electrodes are arranged at regular intervals, and the samevoltage is applied therebetween, thereby forming constant accelerationfields in the direction of the axes. The latter method includes a linearacceleration (linac) method in which ions are transferred along astraight line and accelerated with high-frequency waves. The clustersize can be adjusted by controlling the pressure of gas ejected from anozzle, the pressure of vacuum vessel, the voltage applied to thefilament in the ionization, and the like. The cluster size is determinedby finding the cluster number distribution by mass spectrometry usingthe oscillating quadrupole field or by time-of-flight mass spectrometry,and finding the mean value of the cluster numbers.

The dose of the cluster ions can be adjusted by controlling the ionirradiation time. In this embodiment, the cluster dose is 1×10¹³atoms/cm² to 1×10¹⁶ atoms/cm², preferably 5×10¹³ atoms/cm² or less. In acase of a dose of less than 1×10¹³ atoms/cm², sufficient getteringcapability would not be achieved, whereas a dose exceeding 1×10¹⁶atoms/cm² would cause great damage to the epitaxial surface.

According to this embodiment, as described above, it is not required toperform recovery heat treatment using a rapid heating/cooling apparatusseparate from the epitaxial apparatus, such as RTA/RTO. This is becausethe crystallinity of the semiconductor wafer 10 can be sufficientlyrecovered by hydrogen baking performed prior to epitaxial growth in anepitaxial apparatus for forming the epitaxial layer 20 to be describedbelow. For the conditions for hydrogen baking, the epitaxial growthapparatus has a hydrogen atmosphere inside and it is heated from about1100° C. to 1115° C. to a temperature of 1120° C. to 1150° C. at a rateof 1° C./s to 15° C./s, and the temperature is maintained for 30 s to 1min.

Needless to say, the recovery heat treatment may be performed using arapid heating/cooling apparatus separate from the epitaxial apparatusafter the first step prior to the second step. Even in this case, thecrystallinity can be sufficiently recovered under the conditions of1000° C. or less and less than 10 s which is shorter than conventional.

A silicon epitaxial layer can be given as an example of the secondepitaxial layer 20 formed on the modifying layer 18, and the siliconepitaxial layer can be formed under typical conditions. For example, asource gas such as dichlorosilane or trichlorosilane can be introducedinto a chamber using hydrogen as a carrier gas, so that the sourcematerial can be epitaxially grown on the semiconductor wafer 10 by CVDat about 1000° C. to 1150° C. The epitaxial layer 20 preferably has athickness in the range of 1 μm to 10 μm, more preferably in the range of3 μm to 5 μm. When the thickness is less than 1 μm, the resistivity ofthe second epitaxial layer 20 would change due to out-diffusion ofdopants from the semiconductor wafer 10, whereas a thickness exceeding10 μm would affect the spectral sensitivity characteristics of thesolid-state image sensing device. The second epitaxial layer 20 is usedas a device layer for producing a back-illuminated solid-state imagesensing device.

The second embodiment shown in FIGS. 2A, 2B, 2C, and 2D has a feature inthat not the bulk semiconductor wafer 12 but the first epitaxial layer14 is irradiated with cluster ions. The bulk semiconductor wafer has anoxygen concentration two orders of magnitude higher than that of theepitaxial layer. Accordingly, a larger amount of oxygen is diffused inthe modifying layer formed in the bulk semiconductor wafer than in themodifying layer formed in the epitaxial layer, and the former modifyinglayer traps a large amount of oxygen. The trapped oxygen is releasedfrom the gettering site in a device fabrication process and diffusedinto an active region of the device to form point defects. This affectselectrical characteristics of the device. Therefore, one importantdesign condition in the device fabrication process is to ion-irradiatean epitaxial layer having low solute oxygen concentration and to form agettering layer in the epitaxial layer in which the effect of oxygendiffusion is almost negligible.

(Semiconductor Epitaxial Wafer)

Next, semiconductor epitaxial wafers 100 and 200 produced according tothe above methods will be described. A semiconductor epitaxial wafer 100according to the first embodiment and a semiconductor epitaxial wafer200 according to the second embodiment each has a semiconductor wafer10, a modifying layer 18 formed from a certain element contained as asolid solution in the semiconductor wafer 10, in a surface portion ofthe semiconductor wafer 10, and an epitaxial layer 20 on this modifyinglayer 18 as shown in FIG. 1C and FIG. 2D. Both of themcharacteristically have a concentration profile of the certain elementin the depth direction of the modifying layer 18, having a half width Wof 100 nm or less. Specifically, according to the producing method ofthe present invention, the element constituting cluster ions can beprecipitated at a high concentration in a localized region as comparedwith single-ion implantation, which results in the half width W of 100nm or less. Further, in terms of achieving higher gettering capability,the half width W is preferably 85 nm or less, preferably the lower limitthereof is set to 10 nm. Note that “concentration profile in the depthdirection” herein means a concentration distribution in the depthdirection, which is measured with a SIMS (secondary ion massspectrometer).

The certain element is not limited in particular as long as it is anelement other than the main material of a semiconductor wafer (siliconwhen the semiconductor wafer is a silicon wafer). However, carbon or atleast two kinds of elements containing carbon are preferable asdescribed above.

In terms of achieving higher gettering capability, for both of thesemiconductor epitaxial wafers 100 and 200, the peak of theconcentration profile in the modifying layer 18 lies at a depth within150 nm from the surface of the semiconductor wafer 10. Further, the peakconcentration of the concentration profile is preferably 1×10¹⁵atoms/cm³ or more, more preferably in the range of 1×10¹⁷ atoms/cm³ to1×10²² atoms/cm³, still more preferably in the range of 1×10¹⁹ atoms/cm³to 1×10²¹ atoms/cm³.

The thickness of the modifying layer 18 in the depth direction isdefined as the thickness of a region at depths where a concentrationhigher than the background is detected in the above concentrationprofile, and can be in the range of 30 nm to 400 nm.

According to the semiconductor epitaxial wafers 100 and 200 of thisembodiment, higher gettering capability can be achieved thanconventional, which makes it possible to further suppress metalcontamination.

(Method of Producing Solid-State Image Sensing Device)

In a method of producing a solid-state image sensing device according toan embodiment of the present invention, a solid-state image sensingdevice can be formed on an epitaxial wafer produced according to theabove producing methods or on the above epitaxial wafer, specifically,on the epitaxial layer 20 located in the surface portion of thesemiconductor epitaxial wafers 100 and 200. In solid-state image sensingdevices obtained by this producing method, formation of white spotdefects can be sufficiently suppressed than conventional.

Representative embodiments of the present invention have been describedabove. However, the present invention is not limited on thoseembodiments. For example, two layers of epitaxial layers may be formedon the semiconductor wafer 10.

EXAMPLES Experimental Example 1 Example 1-1

An n-type silicon wafer (thickness: 725 μm, kind of dopant: phosphorus,dopant concentration: 1×10¹⁵ atoms/cm³) obtained from a CZ crystal wasprepared. Next, cluster ions were generated using a cluster iongenerator (CLARIS produced by Nissin Ion Equipment Co., Ltd.) under theconditions shown in Table 1, and the silicon wafer was irradiated withthe cluster ions. After that, recovery heat treatment under theconditions shown in Table 1 was performed using an RTA apparatus(produced by Mattson Thermal Products GmbH) as a heat treatmentsufficient for recovering the crystallinity disrupted by the irradiationwith cluster ions. Subsequently, the silicon wafer was transferred intoa single wafer processing epitaxial growth apparatus (produced byApplied Materials, Inc.) and subjected to hydrogen baking at 1120° C.for 30 s in the apparatus. A silicon epitaxial layer (thickness: 4 μm,kind of dopant: phosphorus, dopant concentration: 1×10¹⁵ atoms/cm³) wasthen epitaxially grown on the silicon wafer by CVD at 1150° C. usinghydrogen as a carrier gas and dichlorosilane as a source gas, therebyobtaining a silicon epitaxial wafer of the present invention.

Examples 1-2 to 1-4

Silicon epitaxial wafers in accordance with the present invention wereprepared in the same manner as Example 1-1 except that the cluster ionirradiation conditions and the recovery heat treatment conditions werechanged as shown in Table 1. In Examples 1-2 and 1-4, recovery heattreatment using an RTA apparatus was not performed. The irradiation withcluster ions was performed at 80 keV/Cluster in Examples 1-1 to 1-4. Theclusters each include three carbon atoms (atomic weight: 12) and threehydrogen atoms (atomic weight: 1). Therefore, the energy received by onecarbon atom was 80×{12×3/(12×3+1×3)}/3=24.6 keV.

Comparative Examples 1-1 and 1-2

Silicon epitaxial wafers according to Comparative Examples were preparedin the same manner as Examples above except that a single-ionimplantation step was performed under the conditions shown in Table 1instead of the step of irradiation with cluster ions. Note that eachsingle ion is implanted into the silicon wafer at an energy of 100 keVin Comparative Examples 1-1 and 1-2.

<Evaluation Method and Evaluation Result>

The samples prepared in Examples and Comparative Examples above wereevaluated. The evaluation methods are shown below.

(1) SIMS Measurement

The samples prepared in Examples and Comparative Examples above weresubjected to secondary ion mass spectrometry (SIMS) to obtain theconcentration profile of the implanted elements. The concentrationprofiles of carbon in Example 1-2 and Comparative Example 1-2 of thesame dose are shown as representative measurement results in FIG. 4.Note that the horizontal axis corresponds to the depth from the surfaceof the silicon wafer. In Example 1-2, the half width was 83.3 nm, andthe peak concentration was 5.83×10¹⁹ atoms/cm³. Meanwhile, inComparative Example 1-2, the half width was 245.9 nm, and the peakconcentration was 1.50×10¹⁹ atoms/cm³. The values of the half widths andthe peak concentrations of the other Examples and Comparative Exampleare shown in Table 1. Further, the peak depths are also shown in Table1.

(2) Evaluation of Gettering Capability

The silicon wafer surface of each sample fabricated in Examples andComparative Examples was contaminated on purpose by a spin coatcontamination method using Ni contaminant liquid and Cu contaminantliquid (1.0×10¹²/cm² each) and was then subjected to heat treatment at900° C. for 30 min. After that, a SIMS measurement was performed. The Niconcentration profiles (FIG. 5) and the Cu concentration profiles (FIG.6) of Example 1-2 and Comparative Example 1-2 are shown with the Cconcentration profiles thereof as representative measurement results.

(3) White Spot Defects

Back-illuminated solid-state image sensing devices were fabricated usingthe samples prepared in Examples and Comparative Examples above. Afterthat, with respect to each of the back-illuminated solid-state imagesensing devices, dark leakage current in photoconductor diodes wasmeasured using a semiconductor parameter analyzing system and theresults were converted into pixel data (data of the number of white spotdefects) to find the number of white spot defects per unit area (1 cm²),thus evaluating the suppression of formation of the white spot defects.The results are shown in Table 1.

(4) Heavy Metal Contamination

The prepared samples were contaminated by a spin coat contaminationmethod using nickel (1.0×10¹² atoms/cm³) and subjected to heat treatmentat 900° C. for one hour, followed by selective etching of the surfacesof the samples. Thus, the defect densities (number/cm²) of the surfacesof the samples were evaluated. The results are shown in Table 1.

(5) Evaluation of LPD Map

Light point defects (LPDs) of the samples fabricated in Examples andComparative Examples were detected using a wafer surface inspectionsystem (SP-1 produced by KLA-Tencor Corporation). The LPD maps ofExample 1-2 and Comparative Example 1-2 are shown as representativemeasurement results in FIG. 7. The number of LPDs in other Examples andComparative Example are shown in Table 1.

TABLE 1 Cluster ion irradiation conditions (Example) Evaluation resultsSingle ion implantation conditions SIMS measurement (ComparativeExample) results Acceler- Peak White ation concen- spot Defect voltageCluster Dose Recovery heat Half- tration Peak defect density Source(keV/ size (atoms/ treatment width (atoms/ position (number/ (number/LPD Element(s) gas Cluster) (number) cm²) temp./time (nm) cm³) (nm) cm²)cm²) (number) Example 1-1 C, H C₆H₁₂ 80 C: 3, H: 3 5.0E14  950° C. × 5 s83.6 5.75E19 80.3 0.4 1.9 2 Example 1-2 C, H C₆H₁₂ 80 C: 3, H: 3 5.0E14— 83.3 5.83E19 80.2 0.4 2.0 1 Example 1-3 C, H C₆H₁₂ 80 C: 3, H: 32.7E14  950° C. × 5 s 80.1 2.90E19 79.8 0.6 2.5 1 Example 1-4 C, H C₆H₁₂80 C: 3, H: 3 2.7E14 — 79.9 2.95E19 79.9 0.7 2.7 1 Comparative C CO₂ 1001 5.0E14 1000° C. × 1 h 245.8 1.47E19 391.5 3.1 4.0 4 Example 1-1Comparative C CO₂ 100 1 5.0E14 — 245.9 1.50E19 388.9 3.5 6.0 10 Example1-2

<Discussion on Evaluation Result>

From the above results, as shown in Table 1, the half width of theconcentration profile of the irradiated elements was smaller in Examplesthan in Comparative Examples. Further, when comparison was made underthe same condition of dose as in Example 1-2 and Comparative Example 1-2(see also FIG. 4), the peak concentrations of Examples were higher thanthose in Comparative Examples. This shows that irradiation with clusterions made it possible to form a modifying layer having more localizedions at a higher density as compared with single-ion implantation.Consequently, the following properties were improved.

First, as shown in FIG. 5 and FIG. 6, large amounts of Ni and Cu weretrapped by the modifying layer in Examples; meanwhile, the amountgettered is obviously small in Comparative Examples. Thus, it isunderstood that higher gettering capability was achieved in Examples.Further, Table 1 shows that formation of white spot defects when forminga solid-state image sensing device can be suppressed, defect density canbe reduced, and LPDs can be reduced, more in Examples than inComparative Examples. Further, in Examples, the crystallinity wassufficiently recovered by hydrogen baking using an epitaxial growthapparatus without performing recovery heat treatment using an RTAapparatus. On the other hand, Comparative Examples needed long timerecovery heat treatment.

Experimental Example 2 Example 2-1

An n-type silicon wafer (thickness: 725 μm, kind of dopant: phosphorus,dopant concentration: 1×10¹⁵ atoms/cm³) obtained from a CZ crystal wastransferred into a single wafer processing epitaxial growth apparatus(produced by Applied Materials, Inc.) and subjected to hydrogen bakingat 1120° C. for 30 s in the apparatus. A first epitaxial layer ofsilicon (thickness: 0.3 μm, kind of dopant: phosphorus, dopantconcentration: 1×10¹⁵ atoms/cm³) was then epitaxially grown on the waferby CVD at 1150° C. using hydrogen as a carrier gas and dichlorosilane asa source gas. Next, cluster ions were generated using a cluster iongenerator (CLARIS produced by Nissin Ion Equipment Co., Ltd.) under theconditions shown in Table 2, and the first epitaxial layer wasirradiated with the cluster ions. After that, recovery heat treatmentunder the conditions shown in Table 2 was performed using an RTAapparatus (produced by Mattson Thermal Products GmbH) as a heattreatment sufficient for recovering the crystallinity disrupted by theirradiation with the cluster ions. The silicon wafer was thentransferred into the epitaxial growth apparatus again, and a secondepitaxial layer was formed on the first epitaxial layer under the sameconditions as the first epitaxial layer, thereby obtaining a siliconepitaxial wafer in accordance with the present invention.

Examples 2-2 to 2-4

Silicon epitaxial wafers in accordance with the present invention wereprepared in the same manner as Example 2-1 except that the cluster ionirradiation conditions and the recovery heat treatment conditions werechanged as shown in Table 2. In Examples 2-2 and 2-4, recovery heattreatment using an RTA apparatus was not performed. Note that the energyreceived by one carbon atom was 24.6 keV in Examples 2-1 to 2-4.

Comparative Examples 2-1 and 2-2

Silicon epitaxial wafers according to Comparative Examples were preparedin the same manner as Examples above except that a single-ionimplantation step was performed under the conditions shown in Table 2instead of the step of irradiation with cluster ions. Note that eachsingle ion is implanted into the silicon wafer at an energy of 100 keVin Comparative Examples 2-1 and 2-2.

<Evaluation Method and Evaluation Result>

The samples prepared in Examples and Comparative Examples above weresubjected to the same five evaluations performed in Example 1, and theresults are shown in Table 2 and FIGS. 8 to 11.

TABLE 2 Cluster ion irradiation conditions (Example) Evaluation resultsSingle ion implantation conditions SIMS measurement (ComparativeExample) results Acceler- Peak White ation concen- spot Defect voltageCluster Dose Recovery heat Half- tration Peak defect density Source(keV/ size (atoms/ treatment width (atoms/ position (number/ (number/LPD Element(s) gas Cluster) (number) cm²) temp./time (nm) cm³) (nm) cm²)cm²) (number) Example 2-1 C, H C₆H₁₂ 80 C: 3, H: 3 5.0E14  950° C. × 5 s84.9 5.78E19 80.4 0.4 1.9 3 Example 2-2 C, H C₆H₁₂ 80 C: 3, H: 3 5.0E14— 84.7 5.74E19 80.3 0.5 2.0 2 Example 2-3 C, H C₆H₁₂ 80 C: 3, H: 32.7E14  950° C. × 5 s 79.9 2.94E19 80.3 0.7 2.5 1 Example 2-4 C, H C₆H₁₂80 C: 3, H: 3 2.7E14 — 79.6 2.97E19 80.1 0.7 2.6 1 Comparative C CO₂ 1001 5.0E14 1000° C. × 1 h 246.8 1.48E19 373.8 3.4 4.2 5 Example 2-1Comparative C CO₂ 100 1 5.0E14 — 247.3 1.51E19 368.4 3.7 6.7 13 Example2-2

Thus, the like results were obtained in Experimental Example 1 where thebulk silicon wafer was irradiated with the cluster ions and inExperimental Example 2 where the epitaxial layer was irradiated with thecluster ions.

INDUSTRIAL APPLICABILITY

The present invention can provide a method of more efficiently producinga semiconductor epitaxial wafer, which can suppress metal contaminationby achieving higher gettering capability.

REFERENCE SIGNS LIST

-   -   100, 200: Semiconductor epitaxial wafer    -   10: Semiconductor wafer    -   10A: Surface portion of semiconductor wafer    -   12: Bulk semiconductor wafer    -   14: First epitaxial layer    -   16: Cluster ions    -   18: Modifying layer    -   20: Second epitaxial layer

1. A method of producing a semiconductor epitaxial wafer, comprising: afirst step of irradiating a semiconductor wafer with cluster ions toform a modifying layer formed from a constituent element of the clusterions contained as a solid solution in a surface portion of thesemiconductor wafer, the semiconductor wafer being composed ofsemiconductor material; and a second step of forming an epitaxial layeron the modifying layer of the semiconductor wafer, to obtain asemiconductor epitaxial wafer having the half width of a concentrationprofile of the constituent element in the depth direction of themodifying layer is 100 nm or less.
 2. The method of producing asemiconductor epitaxial wafer according to claim 1, wherein thesemiconductor wafer is a silicon wafer.
 3. The method of producing asemiconductor epitaxial wafer according to claim 1, wherein thesemiconductor wafer is an epitaxial silicon wafer in which a siliconepitaxial layer is formed on a surface of a silicon wafer, and themodifying layer is formed in a surface portion of the silicon epitaxiallayer in the first step.
 4. The method of producing a semiconductorepitaxial wafer, according to claim 1, wherein after the first step, thesemiconductor wafer is transferred into an epitaxial growth apparatus tobe subjected to the second step without heat treating the semiconductorwafer for recovering the crystallinity.
 5. The method of producing asemiconductor epitaxial wafer according to claim 1, wherein the clusterions contain carbon as a constituent element.
 6. The method of producinga semiconductor epitaxial wafer according to claim 5, wherein thecluster ions contain at least two kinds of elements including carbon asconstituent elements.
 7. The method of producing a semiconductorepitaxial wafer according to claim 1, wherein in the first step, thesemiconductor wafer is irradiated with the cluster ions such that thepeak of the concentration profile of the constituent element in thedepth direction of the modifying layer lies at a depth within 150 nmfrom the surface of the semiconductor wafer.
 8. The method of producinga semiconductor epitaxial wafer according to claim 7, wherein the firststep is performed under the conditions of: the acceleration voltage ofcluster ions is less than 100 keV/Cluster, the cluster size is 100 orless, and the cluster dose is 1×10¹⁶ atoms/cm² or less.
 9. The method ofproducing a semiconductor epitaxial wafer according to claim 7, whereinthe first step is performed under the conditions of: the accelerationvoltage of cluster ions is 80 keV/Cluster or less, the cluster size is60 or less, and the cluster dose is 5×10¹³ atoms/cm₂ or less.
 10. Amethod of producing a solid-state image sensing device, comprisingforming a solid-state image sensing device on the epitaxial layerlocated in the surface portion of the semiconductor epitaxial waferfabricated by the method according to claim
 1. 11. A method of producinga solid-state image sensing device, comprising: preparing asemiconductor epitaxial wafer, comprising: a semiconductor wafer whosematerial is composed of semiconductor material; a modifying layer formedfrom a certain element contained as a solid solution in thesemiconductor wafer, in a surface portion of the semiconductor wafer;and an epitaxial layer on the modifying layer, wherein the half width ofa concentration profile of the certain element in the depth direction ofthe modifying layer is 100 nm or less, forming a solid-state imagesensing device on the epitaxial layer located in the surface portion ofthe semiconductor epitaxial wafer.